1. Technical Field
The present invention relates to mobile telecommunications networks and to the coordination of group calls involving mobile terminals operating in such networks; embodiments of the invention are particularly, but not exclusively, suited to coordinating group calls when members of the group move between cells of the mobile network.
2. Description of the Related Art
Conventional mobile telecommunications networks, for example, digital cellular telecommunication networks according to the GSM Standard (Global System for Mobile Communications) provide voice broadcast services that permit a subscriber to initiate a group broadcast call directed to a group of other subscribers. Such voice broadcast service for GSM systems is specified in GSM Standard 03.68 and a related GSM Standard, namely GSM 03.69.
Typically, a particular group is identified by a group ID. A common control channel (for example the notification channel NCH in GSM) broadcasts a notification message including the group ID and a description of the channel being used for the group call in the network. Thus, users having the correct group ID stored in their mobile terminals, for example stored in the SIM (Subscriber Identity Module) card of a GSM mobile unit, can recognize the notification message, and can then connect to the group call by tuning to the channel described in the notification message. Furthermore in conventional Voice Group Call Service (VGCS) systems a given user can request and obtain an uplink channel, thereby permitting the user to speak to the group. A given mobile user must have a subscription with the group call service provider in order to be permitted to initiate a group call or participate in a group call.
Group call attributes including, for example, a listing of the cells in which the group call is broadcasted (the Group Call Area) are conventionally compiled in a separate network node database, for example the Group Call Register (GCR) in GSM networks. The group call register is typically associated with a Mobile Switching Centre (MSC) in GSM systems.
A particular problem is encountered with group calls when multiple group calls, each having the same group ID, are conducted within overlapping cells. In one conventional arrangement, a mobile station wanting to join a group call will select whichever of the notification broadcast messages that are broadcast on the notification channel in the cell is first received by the mobile device, so that selection of a group to join can be viewed as random. In relation to mobile devices that are already engaged in a group call at the time of moving into an overlapping cell, International patent application having publication number WO0131964 describes a mechanism by which these groups will be merged so as to form a single group call (with a larger number of participants), whilst other known mechanisms constrain mobile devices to remain with the group to which they were associated prior to moving into the cell. In all of these configurations, there is no means of intelligently selecting between group calls: selection is either performed randomly, or hard wired based on initial group designations, or not possible. In addition there is no means of restricting members of a group call: provided a mobile user has a subscription with the group call service provider, its terminal can join a group call.
Another problem is encountered when mobile devices move between cells: subscribers of conventional group call services are typically allowed to move from cell to cell while remaining in the group call, assuming that the group call is broadcast in each new cell into which the subscriber roams. However, if the group call is not being broadcasted in the new cell, then the group call will no longer be available to the subscriber. Further, changing cells often requires a short interruption of the subscriber's reception of the group call until the subscriber's mobile terminal has listened to the notification channel in the new cell and tuned to the proper group call channel in the new cell.